This invention relates to a pigment for shielding of ultraviolet radiation, and more particularly, to a pigment obtained by coating the surfaces of particles of a flaky powder with particles of zinc oxide and of barium sulfate for shielding of, among others, UV-A, a process for manufacturing the same, and the use thereof.
The amount of ultraviolet radiation of a wavelength range of 290 to 400 nm on the ground of the earth comprises about 6% of the light of the sun, of which about 0.5% has a relatively short wavelength in the range of 290 to 320 nm (hereinafter referred to as UV-B), while about 5.5%, or the greater part thereof, has a relatively long wavelength in the range of 320 to 400 nm (UV-A). UV-A passes clouds and window glass more easily because of its long wavelength than ultraviolet radiation having a short wavelength, and human skin is exposed to more UV-A in daily life, and UV-A penetrates into the skin tissue. While UV-B is scattered or absorbed on the surface of the skin and causes sunburn, or like inflammation to the skin, UV-A reaches the inner layer of the skin, and is said to produce in the skin tissue a radical causing aging of the skin by light, such as wrinkling, loosening, and a reduction of elasticity, exerting an adverse effect on the cell membranes, and genes. When intercepting ultraviolet radiation, therefore, it is not sufficient to think merely of shielding of the whole range of ultraviolet radiation, but it is of greater importance and interest to shield UV-A (Journal of Cosmetic Technology, 31, No. 1, pp. 14-30, 1997).
As materials having the power of shielding of ultraviolet radiation, there are, for example, known and commercially available metal oxides such as fine titanium oxide (see e.g. Japanese Patent Publication No. Sho 47-42502), fine iron oxide (see e.g. Japanese Patent Laid-open No. Hei 5-279235), fine zinc oxide (see e.g. Japanese Patent Laid-open No. Sho 60-231607), fine cerium oxide (see e.g. Japanese Patent Laid-open No. Hei 2-823312 and benzotriazole-based, or other organic agents for absorbing ultraviolet radiation. These commercially available agents for shielding of or absorbing ultraviolet radiation have, however, a number of problems including an insufficient shielding power for ultraviolet radiation and a limited scope of use, as will be pointed out below.
Fine titanium oxide, for example, has a low power of absorbing UV-A, and is required to have a larger particle diameter to exhibit shielding UV-A by scattering effect, but its larger particles lose transparency and have a higher degree of whiteness, thereby limiting the scope of its use. Fine iron oxide has a limited scope of use, since it is inferior in its power to shield of ultraviolet radiation versus any other metal oxide, and has a brown color. Fine zinc oxide has a high shielding power for ultraviolet radiation, but its high agglomeration property presents an obstacle to its use. It is necessary to use a special dispersing apparatus having a strong stirring force to disperse zinc oxide into primary particles having a diameter which enables it to shield ultraviolet radiation most effectively, but even if it may be divided into primary particles, it easily agglomerates again.
Fine cerium oxide is so expensive that it can hardly be used except for special purposes.
Benzotriazole-based, and other organic ultraviolet radiation absorbing agents have high absorbing power for ultraviolet radiation, but as they are organic compounds, they are basically lacking in stability, and cannot be expected to produce durable effects. Moreover, the use of any organic ultraviolet radiation absorbing agent as an additive to cosmetics is very strictly restricted from the standpoint of safety.